Amongst the compounds whose use is particularly important in the practical flavoring of foods and drinks in general, as well as in perfumery, there appear a great number of carboxylic acids as well as their ester derivatives. Used as such, or as starting materials for obtaining derivatives of varied nature, these acids or esters form a class of compounds for which a constant effort has been invested by the industry in view of their synthesis. Although the microbial production of certain organic acids has been known for a long time--it is enough to think of the formation of acetic acid upon the bacterial production of vinegar--, to this day, there are still no simple and efficient bioconversion methods for preparing carboxylic acids, methods which are of general use. Yet, it is precisely and mostly in certain particular fields such as the pharmaceutical or the food production ones that such microbiological methods are justified.
In industrial practice there are a great number of methods resorting to Saccharomyces cultures, owing to their availability, their low price and their status as approved organisms for the preparation of food ingredients. Therefore, the use of such a family of microorganisms is well documented in the literature.
The same applies to other families of microorganisms such as those of the Hansenula, Pichia, Candida or Kluyveromyces genus.
On the other hand, oxido-reductive dismutation reactions on various substrates have been described in the prior art. For example, it is known that, under the action of the lactate dehydrogenase, glyoxylic acid is converted into oxalic acid and glycolic acid [Ital. J. Biochem., 20 (1971) 129]. The dismutation of formic aldehyde into formic acid and methanol is also known, which dismutation is induced by the alcohol dehydrogenase [Arch. Biochem. and Biophysics, 141 (1970) 632] as well as by microorganisms from the Pseudomonas putida family [Agr. Biol. Chem., 48 (1984) 2017].
In spite of this, none of the references found mentions or suggests the use of Saccharomyces, or of yeasts from the Hansenula, Pichia, Candida or Kluyveromyces genus, in efficient and advantageous processes for the preparation of carboxylic acids by quantitative oxidation of the corresponding alcohols or aldehydes.
In fact, in the cases where the prior art has taught the use of fungus or yeasts in similar bioconversions of aldehydes or alcohols, the result has indicated that yeasts are either inefficient microorganisms in this type of bioconversion or need to be grown under conditions which are not convenient for large scale exploitation.
For example, K. C. Guven et al., in Parfumerie und Kosmetik 59, 263 (1978), showed that Candida krusei failed to oxidize citral into geranic acid, but produced geraniol instead.
On the other hand, U.S. Pat. No. 5,071,762 which teaches a method for converting short chain alcohols into the corresponding acids by the action of a yeast from the genera Candida, Kluyveromyces, Hansenula, Saccharomyces or Pichia, it was shown that said method required a period of anaerobic fermentation of the yeast, described as physiological or environmental manipulation of the yeast culture, before the bioconversion of the alcohol. In a specific example the authors showed that yeast which had not been manipulated in this manner totally failed to convert butanol into butyric acid. Such a method requires careful monitoring of the amount of ethanol formed during the anaerobic fermentation, which ethanol may easily become toxic to the microorganism being cultivated.